The present invention relates to integrated circuits and a method of packaging integrated circuits and, more particularly, to a method of attaching a die to a substrate.
An integrated circuit (IC) die is a small device formed on a semiconductor wafer, such as a silicon wafer. Such a die is typically cut from the wafer and attached to a substrate or base carrier for interconnect redistribution. Bond pads on the die are then electrically connected to the leads on the carrier via wire bonding. The die and wire bonds are encapsulated with a protective material such that a package is formed. The leads encapsulated in the package are redistributed in a network of conductors within the carrier and end in an array of terminal points outside the package. Depending on the package type, these terminal points may be used as-is, such as in a Thin Small Outline Package (TSOP), or further processed, such as by attaching spherical solder balls for a Ball Grid Array (BGA). The terminal points allow the die to be electrically connected with other circuits, such as on a printed circuit board. IC die can also be attached to other die to form a stacked multichip device.
Referring now to FIG. 1, an enlarged side view of a conventional TSOP 10 is shown. The TSOP 10 includes an integrated circuit or die 12 attached to a paddle 14 with an adhesive material layer 16. The integrated circuit 12 is electrically connected to pins 18 of a leadframe with wirebonds 20. The circuit 12, paddle 14, and wirebonds 20 are encapsulated, such as with a molded plastic 22.
The conventional die attach method covers the paddle 14 with an amount of adhesive material 16 that matches the exact die size only. Thus, as shown in FIG. 2, when the integrated circuit 12 is pressed onto the adhesive material 16, the adhesive material 16 only extends beyond the die size by a distance that is equal to about the height of the die or integrated circuit 12.
FIG. 3 is a top plan view of the integrated circuit 12 attached to the paddle 14 with the adhesive material 16. As can be seen, there is a large portion of the paddle 14 that is not covered by either the integrated circuit 12 or the adhesive material 16.
Some devices are packaged using an exposed pad (EP) design in order to enhance heat dissipation and improve certain electrical characteristics. In an exposed die attach pad design, the die attach pad is soldered directly onto the PCB. That is, the die is attached directly to a metal paddle, which is directly soldered to the PCB. However, special care must be taken when handling and soldering these EP packages. Such EP packages must be handled in a dry environment prior to soldering and exposure to moisture must be minimized prior to assembly to ensure reliable performance. The coverage of the adhesive material 16 during the die attach process is critical to field operation reliability. Voids and variations in thickness of the adhesive material 16 are undesirable. Further, insufficient coverage of the die attach material 16 makes the device susceptible to reliability failures, due to, for example, delamination. Moisture buildup in the die attach adhesive can also cause delamination. The aforedescribed die attach method only marginally achieves Moisture Sensitive Level Three (MSL-3) at 240° C. reflow condition.
It would be desirable to be able to attach a die to a paddle, substrate or carrier with an adhesive such that the device is less sensitive to moisture and more reliable.